This invention relates to an electric component part to be mounted and connected through its lead terminals on the surface of a printed wiring board, and particularly to an electric component part with its lead terminals having a proper shape for the enhanced reliability of connection.
A recent trend of high-density packaging of electric component parts such as integrated circuit (IC) chips, LSI chips, multi-chip hydrid modules and resistor arrays on a printed wiring board is to form such component parts in "flat type" so that their lead terminals can be in flat contact with the surface of a printed wiring board and to solder the lead terminals to the lands on the printed wiring board.
These surface-mounted devices have a typical lead shape as described in, for example, Japanese patent unexamined publication No. 55-113353, and it is shown in FIG. 1. In the figure, reference number 1 denotes a substrate having a circuit function by mounting chip carriers (not shown) accommodating circuit elements, and each of lead terminals 5 consists of a section 2 to be connected to the substrate 1, a section 3 to be connected to a printed wiring board (not shown) and a virtually straight section 4 between the connecting sections. However, when a large number of component parts having the above-mentioned lead terminals are soldered on a printed wiring board to complete a large functional package, a serious problem will arise in the reliability of solder connections between the lead terminals and the printed wiring board. The reason is that a large functional package is usually plugged in through connectors to the slots of the back board, and the plugging force in the assembling operation causes the printed wiring board to warp, which produces a significant external force applied to the solder connections. In testing, a large functional package, it is often subjected to an "in-circuit test" in which only a small part of the printed wiring board is probed, and such a test process also causes a warp of the printed wiring board. In case a printed wiring board has warped, as shown in FIG. 2, from the original shape 6 shown by the solid line to the shape 16 shown by the dashed line, lead terminals 15 located in the central part of the lead alignment are subjected to a compressive stress, while lead terminals 25 located at both end sections are subjected to a tensile stress. Among these stresses the conventional lead shape has been particularly incapable of absorbing the tensile stress, resulting frequently in a breakdown of solder connections and thus in defective connections between the printed wiring board and lead terminals located in the end sections of the lead alignment.